Digital radio frequency transceiver system and method

ABSTRACT

A transceiver architecture for wireless base stations wherein a broadband radio frequency signal is carried between at least one tower-mounted unit and a ground-based unit via optical fibers, or other non-distortive media, in either digital or analog format. Each tower-mounted unit (for both reception and transmission) has an antenna, analog amplifier and an electro-optical converter. The ground unit has ultrafast data converters and digital frequency translators, as well as signal linearizers, to compensate for nonlinear distortion in the amplifiers and optical links in both directions. In one embodiment of the invention, at least one of the digital data converters, frequency translators, and linearizers includes superconducting elements mounted on a cryocooler.

RELATED APPLICATIONS

The present application is a Continuation of U.S. patent applicationSer. No. 12/403,332, filed Mar. 12, 2009, now U.S. Pat. No. 8,260,145,issued Sep. 4, 2012, and is a Continuation of U.S. patent applicationSer. No. 12/403,329, filed Mar. 12, 2009, now U.S. Pat. No. 8,260,144,issued Sep. 4, 2012, and is a Continuation of U.S. patent applicationSer. No. 12/403,326, filed Mar. 12, 2009, now U.S. Pat. No. 8,260,143,issued Sep. 4, 2012, each of which claims the benefit of priority ofProvisional Patent Application 61/035,932, filed, Mar. 12, 2008, theentirety of which are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The present invention is directed towards radio-frequency (RF)transceivers, and more specifically towards transceivers that use directdigitization of RF signals, and digital processing thereof.

BACKGROUND OF THE INVENTION

A simple RF signal consists of a narrowband baseband signal which ismodulated onto a high-frequency RF carrier. A more complex, broaderbandwidth RF signal may consist of multiple narrowband signals modulatedon similar RF carriers. A conventional RF digital data receiver uses ananalog mixer and local oscillator (LO) to translate the signal from thecarrier to a lower frequency, the baseband or intermediate frequency(IF), where it is then typically digitized and further signal extractionis performed in the digital domain. Similarly a conventional RF digitaldata transmitter works in the reverse direction, converting a digitalbaseband or IF signal to analog, followed by mixing it with the LOsignal to upconvert the signal. It is then amplified for transmissionthrough the antenna.

In many wireless communications systems, the antennas are mounted on atall tower, and the digital processing is carried out in basestationslocated remote from the antenna, e.g., on the ground. It is notpractical or efficient to carry GHz-range (microwave) RF signals longdistances over coaxial lines, since signal attenuation is too high.Therefore, the conventional partitioning of the system places amplifiersand analog mixers on the tower with the antenna, as shown in FIG. 1. Thelower-frequency baseband or IF signal can then be carried to and fromthe ground station with minimal attenuation loss using known andstandard coaxial cable. For longer distances or broader bands,low-attenuation optical fibers have been used to carry this baseband orIF signal, typically in digital form.

Optic fiber systems capable of communicating microwave signals areknown. These are typically applied in military radar applications.

A critical requirement in wireless communication systems is maintaininglinearity on both transmission and reception. That is, the analog signalshould not be subject to distortions that result in the alteration ofsignal composition within the band of interest. Spuriousintermodulations from signal components within the band of interest canlimit the useful dynamic range (called the spurious-free dynamic rangeor SFDR) of a receiver or transmitter system. Thus, in a narrow bandsystem, non-linearities which result in spurious energy in frequenciesoutside of the band of interest are generally tolerable, since these arereadily filtered or ignored. On the other hand, as the bandwidth of thesignal of interest grows wider, the possibility of significant spurioussignals in the band of interest due to non-linearities in the signalprocessing and transmission chain grows. These are not readily filteredor ignored, and therefore present limitations of the system.

In a radio system, non-linearities in the signal processing chaingenerally produce spurs or intermodulation products between componentsof the signal. The wider than the band of the signal processed, thegreater the probability for intermodulation components (spurs) which liewithin the band itself, and therefore cannot be eliminated with aband-pass filter. One strategy to avoid misinterpretation of such spursis to employ a deconvolution process in the receiver to predict theeffect of the non-linearity on the signal, and reverse its effects.However, this requires a receiver for the full bandwidth of thetransmitter signal processing chain, which may be untenable. In general,it is preferred to avoid non-linearities, and where present, limit theireffects as much as possible, so that the intermodulation products may betreated as noise.

A number of metrics specifying system performance are available. Forexample, the “Spur-free Dynamic Range” or SFDR, is a commonly usedmetric which presumes a relatively small number of high power signalcomponents which produce intermodulation distortions, and therefore theparameter specifies the dynamic range of the system before spursinterfere with signal interpretation.

Spurious Free Dynamic Range is the usable dynamic range of a systembefore spurious noise interferes or distorts the fundamental signal.SFDR is the measure of the difference in amplitude between thefundamental signals and the largest harmonically or non-harmonicallyrelated spur within the band of operation. A spur is any frequency binon a spectrum analyzer, or from a Fourier transform. Spur-free dynamicrange (SFDR), as generally used, attempts to define receiver dynamicrange in terms of two undesired interferers and the receiver noisefloor. The spur-free dynamic range is the difference in dB between thereceiver noise floor and the level of each of equal-amplitude signalsthat produce an in-band spurious product equal in power to the noisefloor. Generally, the receiver third-order intercept point is used topredict the spurious product, but often the second-order distortiondominates.

The SFDR specification overlooks several important factors whichinfluence dynamic range. First, it attempts to model interference byusing just two (or perhaps up to 4) interfering signals. This overcomessome of the objections to single-tone testing, but the real signalenvironment is usually populated by a multitude of signals. Second, itdoes not reveal the effects of reciprocal mixing or compression like thedesensitization dynamic-range test. Third, it does not effectively testthe effects of receiver input filtering (preselection). Finally, SFDR,as it is ordinarily specified, considers only the third-orderdistortion. In fact, for many receivers, especially those with modestinput filters, the second-order products may dominate.

Another metric employed is the “Noise Power Ratio” or NPR, which modelsthe signal as white noise within a band, and then measures the noisefloor at narrow ranges within the band resulting principally fromintermodulation distortion.

These metrics are therefore extremes of a continuum which seeks tocharacterize a system to determine the impact of intermodulationdistortion on in-band signals; the SFDR measures a specific effect of asmall number, e.g., 2-4 signals, while the NPR measures the statisticaleffect of essentially an infinite number of signals which appear aswhite noise.

In a wideband system, the true SFDR measurement becomes difficult, sinceit may be difficult to determine a worst case effect without testingeach different combination of signals, and the presumptions typicallymade for narrow band systems regarding the dominance of the third orderintermodulation product. On the other hand, white noise itself is astatistical process, and the time of measurement is a relevant factor.For example, over short periods, intermodulation components from variousspectral components within the white noise may cancel or reinforce eachother, leading to a misleading measurement. Since each of these metricis presented as a simple ratio (often specified in decibels), care mustbe exercised in interpretation.

As used herein, the SFDR is intended to encompass the ratio of one ormore respective signal components of equal amplitude within a band andtheir largest in-band intermodulation product, of any order, butexcluding artifacts, such as a superposition of multiple intermodulationproducts. In cases where the SFDR is specified for a composite systemwith band-limiting filters, the input signals may take any value withinthe permissible input range, while the measurements are made afterband-limiting.

The performance of high power amplifiers with many carriers (>10) isnormally tested using a noise power ratio (NPR) measurement technique.In this test, white noise is used to simulate the presence of manycarriers of random amplitude and phase. In a traditional test setup, thewhite noise is first passed through a bandpass filter (BPF) to producean approximately square spectral pedestal of noise of about the samebandwidth as the signals being simulated. This signal is then passedthrough a narrow band-reject filter to produce a deep notch (typ. >50dB) at the center of the noise pedestal. This noise signal is used toexcite the test amplifier, which produces intermodulation distortionproducts that tend to fill in the notch. The depth of the notch at theoutput of the amplifier can be observed with a spectrum analyzer, and isthe measure of the NPR.

NPR can be considered a measure of multi-carrier intermodulation ratio(C/I). NPR differs from multi-carrier C/I in that it is the ratio ofcarrier plus intermodulation to intermodulation (C+I/I). At higherratios (C/I>20 dB), the two measures will approach the same value. Thebandwidth of the noise source should be much wider the bandwidth of theBPF to insure the statistical distribution of the noise power resemblesa random phase multicarrier source. The width of the noise pedestal isusually made equal to bandwidth of the channel under test. The width ofthe notch should be about 1 percent or less of the width of the noisepedestal.

As used herein, the NPR is intended to encompass the dynamic range of awhite noise signal representing the full band range of a permissibleinput signal, with uniform amplitude, with an output signal from whichthe input signal is subtracted. Traditionally, such a measurement isobtained by providing a notch filter at the input for a narrowfrequency, and then measuring the output within that notch band. On theother hand, digital processing techniques permit a more rigorouscharacterization. For example, a notch filter is not necessary; theoutput across the band can be compared with the input, and deviationscharacterized. In that case, the deviation can be expressed as a ratioto the input signal, and for example, a worst case deviation reported asthe noise power ratio. In some cases, especially across a large band,the input amplitude power is not uniform. Since signal from onecommunication stream may interfere with signal from another, which has adifferent power spectrum, the NPR may be functionally specified based onreal signal types, which may differ from equal amplitude noise acrossthe band.

Thus, for example, in a wideband system which handles multiple smallerbands concurrently, a theoretical NPR may not yield fully functionallyvaluable information as compared to signal models. Typical signal typesof interest are cellular communications, which include CDMA (andvariants), GSM, LTE, WiMax, etc. Therefore, in addition tocharacterizing the performance with respect to a white noise across thewide band, it may be useful to determine a bit error rate (BER) of aspecified model signal at a specified attenuation, or alternately, thelimiting attenuation until a specified BER is achieved, in the presenceof other types of signals in the remainder of the band. Since this typeof characterization is communication protocol type-specific, it wouldnot be generally useful for generic systems. Note that in some cases,the relative amplitude of signals within the broadband signal may beadjusted to optimize net performance; for example, if one signalinterferes with that in an adjacent band, and both are not required tooperate at maximum power, then one may be attenuated with respect to theother to achieve acceptable performance for both.

A primary source of nonlinearity in conventional transmission systems isthe power amplifier (PA). In general, highly linear PAs (includingsemiconductor transistor PAs) are energy-inefficient, large and heavy.PAs that are compact and efficient tend to be highly nonlinear. A modernwireless communication system that requires many antennas and PAs on agiven transmission tower may use, or even require the use of, relativelynonlinear PAs.

This nonlinearity limits the performance of the transceiver system, andmust be corrected. Several linearization techniques are known in theprior art. Most of these techniques operate on the baseband signalbefore upconversion (see FIG. 1), or equivalently on the amplitude andphase of the RF signal. One such technique is predistortion, wherein asignal is deliberately distorted in such a way seeking to cancel thedistortion that would be generated by the PA. These linearizationtechniques are generally limited to narrow-band signals, and furthermorethe nonlinearities tend to become worse as the bandwidth increases.

It is well known in the prior art that there are important potentialadvantages to combining multiple RF signals in nearby bands to createsingle broadband signal. For example, this could substantially reducethe number of RF components required. However, the problems associatedwith nonlinear PAs and the inability to sufficiently correct theirdistortions severely limit the bandwidth of such a combined signal toless than about 100 MHz. Conventional technology does not have asolution to this problem, which makes broader band RF systemsimpractical.

Recently, a superconducting electronic technology (known asrapid-single-flux-quantum logic or RSFQ) has been demonstrated that canprovide direct digitization of RF signals, as well as ultrafast digitalprocessing at clock rates up to 40 GHz, with rates up to 100 GHzexpected in the near future. This permits representation of RF signalsin a directly digitized format, at broadcast frequencies, referred to as“Digital RF”™ (Hypres Inc.), wherein the sample rate is much larger thanthe carrier frequency, e.g., above the Nyquist rate. This provides apromising approach to very broadband multi-carrier RF communicationsignals (See, U.S. Pat. Nos. 7,280,623, 7,365,663, 7,362,125, 7,443,719,expressly incorporated herein by reference). However, RSFQ is anultra-low-power technology (sub-mV level signals) that requires coolingto temperatures of 4 K (−269 C). Existing refrigeration technology makesit impractical to place these cryocooler systems on cellular towers, sothey must be placed in ground stations. In that case, transmissionlosses of the RF signals between the ground and the tower largelyeliminate the potential performance advantages. On the other hand, ifthe existing cryocoolers were sought to be placed on the tower, thiswould increase wind loading and power consumption on the tower, andpotentially lead to increased maintenance costs due to potentialservicing of the cryocooler in an inaccessible location. Since the towerelectronics are standardized for various applications, even in caseswhere the tower components are accessible or increased maintenance andservice permissible, the lack of economies of scale still make suchinstallations infeasible.

Optical fibers would seem to be a promising alternative, sinceattenuation losses on fibers are quite small. However, the electro-optic(E/O) and opto-electronic (O/E) converters for prior-art multi-GHzanalog signals had severe limitations in performance, again greatlyreducing the performance advantages of the superconducting system. Theseconverter limitations reflect both limits in bandwidth and limits inlinearity. It is difficult to simultaneously achieve high bandwidth andhigh linearity, particularly in a reliable and inexpensive manufacturedproduct. See, for example, U.S. Pat. No. 7,424,228, expresslyincorporated herein by reference. There are several known methods forreducing nonlinearity (see, for example, U.S. Pat. No. 7,426,350,expressly incorporated herein by reference), but this remains adifficult problem.

The distinctions between digital and analog E/O (and O/E) modulatorsshould be appreciated, which are directly analogous to the distinctionsbetween digital and analog amplifiers. Digital amplifiers and modulatorsare nonlinear one-bit threshold devices, where only bit-errors areimportant. In contrast, analog amplifiers and modulators are generallyrequired to be linear over a wide dynamic range, while nonlinearitiesgenerate intermodulations that reduce the useful spur-free dynamic range(SFDR). Digital optical modulators are long established for high-speeddigital optical communications. In contrast, the technology of linearoptical systems at microwave frequencies (microwave photonics orradio-over-fiber, or “RoF”) has been developing only over the pastdecade, with further improvements in performance continuing. For E/Oconverters, direct modulation of diode lasers is typically used for RFfrequencies below a few GHz, while electro-optic interferometricmodulation is used for higher frequencies. Photodiodes are used for O/Econverters. The nonlinearity associated with such an optical linkincreases sharply with increasing bandwidth. A typical optical linkspecification may be expressed by SFDR=100 dB/B^(2/3), corresponding to3^(rd)-order intermodulation, where B is the bandwidth in dB-Hz. For a10 GHz signal, corresponding to 100 dB-Hz, the link SFDR would bereduced to 33 dB. This would likely be quite sufficient for a digitalsignal, but would be too low for a high-dynamic range analog signal.

The low-power aspect of this superconducting technology requires the useof high-gain, high-bandwidth semiconductor amplifiers, which are nearthe limits of semiconductor technology, and commercial product isgenerally unavailable. Various non-ideal aspects of these amplifiers,especially nonlinear distortion, tend to degrade the performance of theoverall system. Very recently, the same superconducting RSFQ technologyhas demonstrated the capability to provide very broadband digitalpredistortion directly on the digitized RF signal, for a digital-RFtransmission system with a nonlinear PA. See, e.g., U.S. Pat. No.7,313,199, expressly incorporated herein by reference. This can achievemuch greater improvement in broadband system SFDR than would be possibleusing conventional baseband predistorters, and might be used to makethese broadband systems practical. However, the link between the groundand tower units remains a problem.

SUMMARY OF THE INVENTION

One aspect of the present invention addresses these shortcomings andseeks to provide a hybrid system that combines these three technologiesof superconducting RSFQ circuits, microwave-frequency optical links, andbroadband semiconductor power amplifiers, in a way that builds onsuperior features of each and corrects the non-idealities that limitpractical performance.

A new system architecture is presented for an RF transceiver in awireless communication base station, wherein ultrafast multi-GHz digitaldata conversion and processing are carried out in a central groundstation, and analog-RF amplification is carried out in tower-mountedunits close to antennas. No analog mixing is necessary in the tower,since the signal communicated to the tower is a direct representation ofthe radio frequency signal to be transmitted. The ground station islinked to the tower units by means of broadband RF signals carried onoptical fibers in either analog or digital format, in contrast toprior-art systems that carry only baseband (or IF) signals. High-speeddigital processing of the full digital-RF signal is used to correct fornonlinearities in amplifiers and electro-optic converters in bothtransmitter and receiver, as shown in FIG. 2. In a preferred embodimentof the invention, the high speed digital processing and data conversionare carried out using a superconducting RSFQ processor, operating atcryogenic temperatures in a cryocooler, as shown in FIG. 3, whichtherefore provides the cryocooler and superconducting components in thebase station, and not on the tower. (It should be noted that the “basestation” may in some cases be physically integrated with thetower/antenna complex, though the functions remain separated asdiscussed herein).

The technology provides a new architecture for a radio system,leveraging the ultrafast digital-to-analog converter, analog-to-digitalconverter, and digital signal processing technologies presented herein.

In the case of low temperature superconductors, a cryocooler isrequired, which consumes energy to achieve cryogenic temperatures, i.e.,<10K, and has a physical size. In addition, the cryocooler presents apossible service and maintenance issue, and a preference may thereforeexist to have such a cryocooler located off an antenna, and, forexample, in a base station located on the ground, or rooftop, orotherwise some distance from the antenna structure. On the other hand,the transmit channel of the radio system typically has at least a poweramplifier located in close proximity to the antenna, and the receivechannel has at least a receive amplifier, typically a low noiseamplifier, located in close proximity to the antenna. Therefore, aspectsof the present technology address efficient and effective communicationof signals from the antenna electronics system to a remote location.

A particular issue addressed in accordance with the present technologiesis an avoidance of frequency translation between an analog radiofrequency interface and digital electronics, which leads to distortionand degradation of wideband radio frequency signals. For example, awideband radio frequency signal may be greater than 19.5 MHz (20 MHzband spacing less guard-band), for example, 40 MHz (two adjacent 20 MHzbands) or 60 MHz (3 adjacent bands). Indeed, using high speed digitalelectronics, it is possible to process, at native frequency, signalshaving bandwidths of in excess of 100 MHz. A particular advantage ispossible where the antenna system communicates on multiple discontiguousbands, for example, 3 bands within a 120 MHz range, since a systemimplemented in accordance herewith would be agnostic to the bandselection, and therefore be more versatile and adaptive.

For high fidelity, large bandwidth radio frequency signals, thecommunication medium of choice is an optic fiber, though coaxial orother impedance controlled electrical cable may be employed. However,each of these suffers from distortion and attenuation of the signals,and requires amplification. It is noted that the issues for transmit andreceive channels are not symmetric, and thus each issue is preferablyconsidered separately. In particular, according to one aspect of theinvention, the substantial high speed digital electronics are locateddistant from the antenna electronics system. Therefore, the receivechannel may communicate an analog signal to the remote location. On theother hand, the transmit channel may communicate an analog or digitalsignal to the antenna electronics system, and advantages accrue if thecommunication is digital, since attenuation and distortion in thecommunications medium may be completely corrected; however, this schemethen requires a digital to analog converter operating at the data ratelocated within the antenna electronics package.

Three options are generally available for a transmit channel: a firstoption provides an analog signal communicated from the remoteelectronics station to the antenna electronics system, for poweramplification and transmission. In this case, the remote electronics canpredistort the signal to compensate for various types of distortion inthe downstream electronics chain, though there is limited capability tocompensate for intermodulation distortion. Thus, on this embodiment, thevarious elements of the communication link and transmission-side antennaelectronics system should have high linearity.

A second option provides a baseband or intermediate frequencycommunication link from the remote electronics station, with frequencytranslation at the antenna electronics system. This option correspondsto a more typical cellular radio system. While employing lower frequencycommunications between the remote electronics system and antennaelectronics system permits use of traditional coaxial cable, suchsystems have limited bandwidth.

A third option provides a digital communication between the remoteelectronics and the antenna electronics system. In this embodiment, wepresume that the high speed digital electronics processing capabilitiesare not available in the antenna electronics system (though in somecases, they are, avoiding the need for significant communicationsbetween the modules), and therefore a fully processed, digital streamwhich is oversampled with respect to the radio frequency signal to beemitted from the antenna, and thus the antenna electronics system needonly convert the digital stream into a corresponding signal, eitherbefore, during or after amplification. For example, the digital streammay be communicated through an optic fiber link, and converted from adigitally modulated optic signal to digital pulses. The digital pulses,if oversampled, may then be low pass filtered to produce an analog radiofrequency waveform, which may then be amplified by an analog poweramplifier.

According to another scheme, the digital stream may be communicatedthrough an optic fiber link, and converted from a digitally modulatedoptic signal to digital pulses. The digital pulses are then used todrive a power digital-to-analog converter, which produces a high power,e.g., 10 Watt, analog radio frequency signal from the digital stream. Inthis case, a multibit digital signal may be provided, thus reducing thedegree of oversampling required for high fidelity, and potentiallyfacilitating the high power digital to analog converter operation. Thus,in contrast to an ideal digital to analog converter, the slew rate ofthe converter may be limited, thus permitting the higher order bitdrivers, which correspond to higher power, to switch slower than thelower order bits, which correspond to lower power. Indeed, since thecharacteristics of the power digital to analog converter may be known bythe remote electronics system, certain non-idealities may beprecompensated. Based on the availability of precompensation, a matchedpower digital to analog converter may be provided such that theavailable compensation better matches the distortion. Thus, for example,a 4 bit dynamic range converter may have 5 or more digital inputsignals. In this way, cost and/or power consumption or other operatingcharacteristics may be optimized.

As will be discussed further below, one or more upstream channels may beprovided from the antenna electronics system to the remote electronicssystem. A principal application is for communication of a representationof a received radio frequency signal or signals. However, a feedbacksignal, such as a representation of the power amplified radio frequencysignal transmitted or to be transmitted by the antenna may be returnedto the remote electronics system, thus permitting a feedback-controlledpre-compensation of the signal communicated to the antenna electronicssystem, or more generally, an adaptive system capable of responding tochanges in the system or environment to control the system operation.Thus, operating temperature may affect the output of the antennaelectronics system. Internal temperature compensation may be provided,as a known option. On the other hand, temperature compensation (andother types of compensation) may be absent in the power amplificationsystem, and these functions implemented digitally in the remoteelectronics system. This, in turn, permits, for example, a moreefficient and/or less expensive power electronics design, and indeedpotentially permits use of a power electronics module which requiresunder all or substantially all operating conditions, precompensation ofthe signal in order to produce a usable output signal.

In accordance with an aspect of the technology, the distortion, measuredas a Digital Noise Power Ratio (DNPR) of the transmitted signal, definedas the maximum RMS distortion amplitude of the output within a narrowrange, e.g., 1 kHz (yielding a DNPR_(1kHz)), over the entire bandwidthof a band, with respect to the RMS amplitude of a white noise signalwith uniform RMS amplitude, extending over the entire band presented atthe input, when measured over an arbitrarily long duration. Themeasurement may be made, for example, by digitizing the output andcomparing it to the input with a digital signal processor, and thereforewithout requiring real filter implementations applied to the input oroutput signals. Typically, the effects of phase delay and group delayare excluded, and thus the distortion measurements are delaycompensated, for example by use of an autocorrelator. Of course, therange of measurement can vary in accordance with the application.Typically, a transmitter system according to a preferred embodiment willprovide a DNPR_(1kHz) of larger than 40 dB over a bandwidth of 120 MHz,e.g., from 2.4-2.52 GHz, more preferably larger than 50 dB, and perhapsextending up to 96 dB or greater.

The present technology also provides a receiver system. In this case, aradio frequency receiver will comprise an antenna or input from anantenna, a low noise amplifier, which may be cooled, for example <100K,and indeed, may be associated with high temperature superconductorfilters operating at <78K, though will typically not be low temperaturesuperconductor components. The signal from the low noise amplifier maydrive an electrooptic converter, to thereby communicate an opticalsignal modulated with an analog radio frequency waveform, at its nativeradio frequency, to a remote location. As discussed above, this sameoptic link system, or a corresponding optic link, may also carryfeedback signals, such as may be used to control over a transmittersystem.

A receiver system in accordance with this embodiment does not requirefrequency translation components, and thus no mixer is used. Therefore,the level of spurious intermodulation components generated by thereceiver may be minimized. Further, by ensuring that the pattern ofintermodulation components with significant power within the band isrelatively simple, digital deconvolution can be employed, if necessary.Digital deconvolution may also be employed in the receiver system tocompensate for distortion in the corresponding signal transmitter. It isthus important to realize that while most radio systems treatintermodulation distortion as noise, and thus additive to the noisefloor, these signal components are not stochastic, and to some extentthat distortion can be reversed, in contrast to stochastic noise, whichis truly random.

The optic link conveys the signal to the remote location, where anultra-high speed analog-to-digital converter operates to digitize theradio frequency signal at its native frequency. In such a system, theSFDR can reach >90 dB (1 Hz bandwidth), even for wideband applications,from antenna to analog-to-digital converter, an amplified fiber opticlink has an SFDR of, for example, >106 dB (in 1 Hz). Theanalog-to-digital converter and associated elements have, for example,an SFDR of −98.5 dBc over a 39 MHz bandwidth for a 20 GHz signal.

Other architectures may be used for the receiver as well, for examplewith the analog-to-digital converter located proximate to the antenna,frequency down-conversion, and the like.

A radio system is therefore provided, comprising an optoelectronicreceiver adapted to receive an input from an optic communication mediumcomprising a digital domain representation of a radio frequency signalat a radio frequency data rate of at least 1.4 GHz; a converter adaptedto receive a digital electronic signal from the optoelectronic receiverand convert the digital domain representation of the radio frequencysignal into an analog domain radio frequency signal of at least 700 MHz;and an interface to an antenna adapted to transmit the radio frequencysignal. It is noted that the 700 MHz band, which nominally begins at 698MHz, is intended for new radio frequency mobile services, such as “LongTerm Evolution” (LTE). The system, of course, may operate at higherfrequencies, and for example, the converter, and other high speeddigital electronic circuitry may operate at up to 40 gigasamples persecond, or higher, permitting direct synthesis, without analogup-conversion, of signals of up to 20 GHz, or higher, and at lowerfrequencies, provides for greater oversampling opportunity. For example,the bands covered notably include the 700 MHz band, the 900 MHz band,1.2-1.5 GHz bands, 1.8-1.9 GHz bands, 2.4-2.5 GHz bands, and the like.By avoiding analog mixers, a significant source of intermodulationdistortion is avoided, thus permitting operation with a high spur freedynamic range. The interface may thus produce the analog domain radiofrequency signal corresponding to the digital domain representationsignal having a spur-free dynamic range of at least 40 dB over abandwidth of at least 19 MHz. This 19 MHz represents, for example, aband having a band spacing of 20 MHz with a 0.5 MHz guard band on eachside. Because the guard band does not generally include informationsignals, it may be acceptable for some intermodulation products or otherspurious radiation to be present, though regulations typically controlsuch emissions. Where a set of contiguous bands are combined, the entireblock of frequencies may be treated as a single information signal.

The optoelectronic receiver and/or transmitter may comprise a multiplextransmitter and/or receiver, adapted to extract and/or injectinformation modulated on one of a plurality of information-carryingchannels of the optic communication medium, substantially withoutinterference with another of the information-carrying channels of theoptic communication medium. For example the optic communication may bewavelength division multiplexed. In some cases, a digital signal iscommunicated on the optic communication medium, e.g., fiber optic, andvarious digital multiplexing schemes may be employed.

The converter, and other components may have an associatedintermodulation distortion of the analog domain radio frequency signaland the digital domain representation of the radio frequency signal maybe predistorted in a manner to compensate for at least a portion of theassociated intermodulation distortion, to increase an effectivespur-free dynamic range of the radio system. The predistortion may beadapted to improve the effective spur-free dynamic range by at least 3dB, and more preferably by at least 6 dB. Larger degrees of reductionmay be possible.

The system may further have an interface to an antenna adapted toreceive a radio frequency signal of at least 700 MHz; a low noiseamplifier and filter adapted to amplify the received radio frequencysignal within a band; and an electrooptic transmitter adapted totransmit a signal corresponding to the received radio frequency signalwithin a band.

The converter may have an associated intermodulation distortion of theanalog domain radio frequency signal and the digital domainrepresentation of the radio frequency signal may predistorted in amanner compensate for at least a portion of the associatedintermodulation distortion, to reduce an effective amplitude of at leastone intermodulation product present in the analog domain radio frequencysignal by at least 3 dB, and more preferably, by 6 dB. The system mayfurther comprise an interface to an antenna adapted to receive, amplifyand filter a received radio frequency signal; and an electroopticconverter adapted to generate an optic signal corresponding to thereceived radio frequency signal. A digital processor may be provided,adapted to increase an effective spur-free dynamic range of the receivedradio frequency signal by at least 3 dB, or at least 6 dB. Likewise, adigital processor may be provided, adapted to predistort the digitaldomain representation to increase an effective spur-free dynamic rangeof the analog domain radio frequency signal by at least 3 dB or at least6 dB. The digital processor may comprise at least two Josephsonjunctions, and thus be part of a superconducting electronic circuit,such as a set of logic gates.

A radio system is also provided, comprising an optoelectronic receiveradapted to receive a signal from an optic communication mediumcomprising a radio frequency signal having at least one significantinformation-bearing component having a frequency of at least 698 MHz,and ranging, for example, from the 700 MHz band to the X band (7-12.5GHz); an amplifier adapted to amplify the signal and generate a powerradio frequency signal, substantially without frequency translation; andan interface to an antenna adapted to emit the power radio frequencysignal, wherein the signal is predistorted to increase an effectivespur-free dynamic range of the power radio frequency signal by at least3 dB, and preferably at least 6 dB, by reducing a power of at least oneintermodulation signal with respect to a corresponding signal absentpredistortion. A transmitter may be provided, adapted to communicate afeedback signal corresponding to the power radio frequency signal,wherein the predistortion is based on at least the feedback signal. Thepower radio frequency signal is preferably generated with has aspur-free dynamic range of at least 40 dB over a bandwidth of at least19 MHz, and preferably over 39 MHz, 59 MHz, or larger bandwidth.Likewise, the spur free dynamic range may be greater than 40 dB, and forexample, may be 45 dB or more. The signal may have at least onesignificant information-bearing component having a frequency of at least1.5 GHz, for example 1.8 GHz or 2.4 GHz.

The amplifier may comprise a power digital to analog converter, whichdirectly generates an analog radio frequency signal from the at leastone significant information-bearing component, having an amplitude of atleast 27 dBm, and wherein the power radio frequency signal has anaverage power of at least 30 dBm.

The radio system may further comprise an interface to an antenna adaptedto receive a radio frequency signal of at least 698 MHz; a low noiseamplifier and filter adapted to amplify and filter the received radiofrequency signal; and an electrooptic transmitter adapted to transmit asignal corresponding to the amplified and filtered received radiofrequency signal.

A digital processor may be provided, adapted to predistort the signal toincrease an effective spur-free dynamic range of the power radiofrequency signal by reducing a power of at least one intermodulationproduct. Preferably, a largest amplitude intermodulation product (spur)is reduced in amplitude, but this is not always the case. Likewise,preferably the amplitude of a plurality of intermodulation products isreduced.

A radio transmitter is also provided, comprising an optoelectronicreceiver adapted to receive an input from an optic communication mediumcomprising a digital domain representation of a radio frequency signalat a bit rate exceeding 2 Gigasamples per second and produce a digitalelectronic signal therefrom; a converter adapted to receive the digitalelectronic signal from the optoelectronic receiver and produce aninformation-bearing analog domain radio frequency signal of at least 1GHz, substantially without frequency translation of the digital domainrepresentation; and an interface to an antenna adapted to emit the radiofrequency signal.

A radio system is further provided, comprising a receiver adapted toreceive an input from a communication medium comprising a discrete time,quantized amplitude modulated representation of a signal, at anintersymbol modulation rate exceeding 2 giga-symbols per second; aconverter adapted to receive the signal from the receiver and convertthe signal into an analog domain radio frequency signal of at least 698MHz, for example, greater than 1.5 GHz or 2.4 GHz and a bandwidth of atleast 59 MHz; and an interface to an antenna adapted to emit the radiofrequency signal, wherein the analog domain radio frequency signal hasintermodulation products not expressly defined by the signal having anamplitude of at least −40 dB with respect to the signal; and a digitalpredistorter adapted to precompensate the input to reduce an amplitudeof at least one intermodulation product by at least 3 dB, and preferablyby at least 6 dB.

A wideband radio receiver is also provided, comprising at least onetower unit mounted on a tower, each tower unit comprising an antennaadapted to receive an incident wideband frequency distribution radiosignal, an amplifier adapted to amplify the wideband frequencydistribution radio signal, and an electro-optic converter adapted toconvert the amplified wideband frequency distribution to an analogoptical signal; an optical fiber communications link adapted tocommunicate the analog optical signal; and a base unit remote from thetower, adapted to receive the analog optical signal from the opticalfiber communications link, comprising an opto-electric converter adaptedto convert the optical signal received from the optical fiber link to ananalog radio signal with a wideband frequency distribution correspondingto the wideband frequency distribution of the signal incident on theantenna, an analog-to-digital converter adapted to directly digitizesthe entire wideband distribution, and a digital linearizer, adapted todigitally compensate the digitized wideband distribution for at leastnonlinear distortion, to substantially reconstruct a digitally sampledequivalent of the frequency distribution of the signal incident on theantenna, having a spur-free dynamic range greater than a spur-freedynamic range of the optical signal.

A system is also provided for extracting information from a radiofrequency signal, comprising: an input adapted to receive a radiofrequency signal comprising information and significant radio frequencyenergy above 698 MHz; an amplifier adapted to amplify the radiofrequency signal, wherein a bandwidth of the radio frequency signalcomprising information is at least 19 MHz, and for example, 39 MHz, 59MHz, 79 MHz, 100 MHz, 120 MHz, or larger bandwidth; an analogcommunication system having a communication length in excess of 1 meter,adapted to convey the amplified radio frequency signal, wherein theconveyed radio frequency signal has an associated distortion withrespect to the radio frequency signal; a digitizer having a samplingrate in excess of 2 GHz, for example 10 GHz, 20 GHz or 40 GHz, adaptedto digitize entire bandwidth of the conveyed radio frequency signal; anda digital signal processor adapted to correct at least a portion of theassociated distortion, operating on the digitized conveyed radiofrequency signal from the digitizer having a sampling rate in excess of2 GHz.

A radio system is further provided, comprising: an optoelectronicreceiver adapted to receive an input from an optic communication mediumcomprising a radio frequency signal having a bandwidth of at least 59MHz at a band frequency higher that 698 MHz; an amplifier adapted togenerate a power radio frequency signal corresponding to the radiofrequency signal having an average power of at least 36 dBm (˜4 watts),and for example having an average power of at least 40 dBm (10 watts);and an interface to an antenna adapted to transmit the power radiofrequency signal, wherein the power radio frequency signal has adistortion with an error vector magnitude of at least 1% with respect tothe radio frequency signal communicated by the optical communicationmedium. A digital signal processor may be provided, for example, clockedat least 5 GHz, and for example at 10 GHz, 20 GHz, 40 GHz, or faster,adapted to precompensate a digital data stream representing the radiofrequency signal for at least a portion of the distortion. The errorvector magnitude of the distortion may be, for example, at least 2% orat least 4%, and further comprising a digital signal processor adaptedto precompensate a digital data stream representing the radio frequencysignal to achieve a reduction in distortion of at least 3 dB, andpreferably by at least 6 dB. For example, the error vector magnitude ofthe distortion is at least 4% and a digital signal processor is providedadapted to precompensate the radio frequency signal to achieve areduction in error vector magnitude of at least 3%.

This compensation capability (predistortion) therefore allows the use oflower cost, lower power, higher power efficiency, or other advantageoustypes of devices, which may be mounted, for example, on an antennatower.

A radio system is further provided, comprising a digital signalprocessor adapted to process a digital signal representing informationto be communicated through a radio frequency transmission having asignificant frequency component exceeding 698 MHz, having at least oneinput for receiving information, at least one logic unit adapted todefine digital codes representing a modulated radio frequency signaloversampled with respect to the radio frequency transmission, and atleast one digital logic unit adapted to produce a processedrepresentation of the defined digital codes, by at least one of:

(a) predistorting the defined digital codes to increase a spur freedynamic range of the radio frequency transmission, based on a predictednon-linear distortion of a radio frequency transmitter;

(b) combining at least two sets of defined digital codes, each beingoversampled with respect to a radio frequency transmission frequency inexcess of 698 MHz;

(c) processing a signal representing at least one set of defined digitalcodes to reduce a peak to average power ratio of the radio frequencytransmission after conversion to an analog radio frequency signal;

(d) processing the defined digital codes to provide source-encoding of aset of signals for driving a multiple antenna array, wherein preferablyat least two antennas of the multiple antenna array receive respectivelydifferent signals; and

an output, adapted to present the processed representation.

The at least one digital logic unit may perform any two, or all threefunctions, in addition to other functions, in any order.

An optoelectronic modulator may be provided, adapted to communicate theoutput through an optic communication medium. A digital to analogconverter may be provided, adapted to convert the defined digital codesinto an analog radio frequency signal. Likewise, both a digital toanalog converter adapted to convert the defined digital codes into ananalog radio frequency signal, and an optoelectronic modulator adaptedto communicate the analog radio frequency signal through an opticcommunication medium may be provided. The radio system may furthercomprise an optoelectronic demodulator, and a power amplifier to amplifythe analog radio frequency signal to a power of at least 27 dBm, whereinthe defined digital codes are predistorted to reduce an effect of anon-linear distortion of at least the optoelectronic modulator, opticcommunication medium, the optoelectronic demodulator, and the poweramplifier. An optoelectronic modulator may be provided, adapted tocommunicate the output as a digital communication through an opticcommunication medium, an optoelectronic demodulator adapted to produce acommunicated digital signal representing the output at a remote locationfrom the optoelectronic modulator, and a digital to analog converteradapted to convert the communicated digital signal representing theoutput into an analog radio frequency signal. The analog radio frequencysignal may be provided with a power of at least 27 dBm, and wherein thedefined digital codes are predistorted to increase a spur free dynamicrange of the radio frequency transmission, to reduce an effect of anon-linear distortion of the analog radio frequency signal. The defineddigital codes may be processed to reduce a peak to average power ratioby at least 1 dB, and for example, 1.5 dB, 2 dB, 2.5 dB, or 3 dB. Forexample, at least two sets of defined digital codes are combined, andthe combined sets of digital codes are processed to reduce a peak toaverage power ratio by at least 1 dB. The output may presents theprocessed representation an analog domain radio frequency signal havinga spur-free dynamic range of at least 40 dB over a bandwidth of at least19 MHz.

The digital logic unit may comprise at least one look-up tableresponding to look-up requests at a rate of at least 698 megasamples persecond, and may provide a plurality of look-up tables operating intandem, for example to implement a proportional-integral-differentialprocessing algorithm in conjunction with a delay logic device, anintegrator logic device, and a differentiator logic device.

The radio system may further comprise a digital crosspoint switchmatrix. The radio system may further comprise a first optoelectronicmodulator adapted to generate an optical communication of the output; aninterface to an antenna adapted to receive a radio frequency signal ofat least 698 MHz; and a second optoelectronic modulator adapted tooptically communicate a signal corresponding to the received radiofrequency signal. The defined digital codes may be processed by thedigital logic unit to reduce an amplitude of at least oneintermodulation product by at least 3 dB, preferably by at least 6 dB,based on a digital model of a non-linear process operating on theoutput. The digital signal processor may comprise an input adapted toreceive a feedback signal representing a distorted of the output, andwherein the predistortion is responsive to the feedback signal. Thedigital signal processor comprises at least two Josephson junctions.Likewise, the digital signal processor may comprise an activesuperconducting logical device.

The output may represents at least one significant information-bearingcomponent having a frequency of at least, for example, 1.0 GHz, or 1.5GHz, or 2.0 GHz, or 3 GHz, or higher.

The radio system may further comprise an optoelectronic transmitteradapted to communicate the output as a signal in an optic communicationmedium; an optoelectronic receiver adapted to receive the signal fromthe optic communication medium comprising information defining a radiofrequency signal having at least one significant information-bearingcomponent having a frequency of at least 698 MHz; an amplifier adaptedto amplify the signal generate a power radio frequency signal,substantially without frequency translation; and an interface to anantenna adapted to emit the power radio frequency signal, wherein thesignal is predistorted to increase an effective spur-free dynamic rangeof the power radio frequency signal by reducing a power of at least oneintermodulation signal with respect to a corresponding signal absent thepredistortion. A transmitter may be provided, adapted to communicate afeedback signal corresponding to the power radio frequency signal to thedigital signal processor, wherein the predistortion is based on at leastthe feedback signal. The power radio frequency signal may have aspur-free dynamic range of at least 40 dB over a bandwidth of at least19 MHz, and for example over a bandwidth of 39 MHz or 59 MHz. Theamplifier may comprise a power digital to analog converter, whichdirectly generates an analog radio frequency signal from the at leastone significant information-bearing component, having an amplitude of atleast 27 dBm, and wherein the power radio frequency signal has anaverage power of at least 30 dBm. The predistortion preferably improvesthe effective spur-free dynamic range by at least 3 dB, and morepreferably by at least 6 dB.

A radio transmitter is provided, comprising a digital signal processoradapted to produce a digital domain representation of a radio frequencysignal, sampled at a rate of at least 1.5 gigasamples per second; anoptoelectronic transmitter adapted to communicate over an opticcommunication medium the digital domain representation of a radiofrequency signal at a rate of at least 1.5 gigasamples per second; anoptoelectronic receiver adapted to receive an input from the opticcommunication medium and produce a digital electronic signal therefrom;a converter adapted to receive the digital electronic signal from theoptoelectronic receiver and produce an information-bearing analog domainradio frequency signal having significant frequency components exceeding698 MHz; and an interface to an antenna adapted to emit the radiofrequency signal.

The digital signal processor may be adapted to process a digital signalrepresenting information to be communicated through a radio frequencytransmission having a significant frequency component exceeding 698 MHz,having at least one input for receiving information, at least one logicunit adapted to define digital codes representing a modulated radiofrequency signal oversampled with respect to the radio frequencytransmission, and at least one digital logic unit adapted to produce aprocessed representation of the defined digital codes, by at least oneof:

(a) predistorting the defined digital codes to increase a spur freedynamic range of the radio frequency transmission, based on a predictednon-linear distortion of a radio frequency transmitter;

(b) combining at least two sets of defined digital codes, each beingoversampled with respect to a radio frequency transmission frequency inexcess of 698 MHz;

(c) processing a signal representing at least one set of defined digitalcodes to reduce a peak to average power ratio of the radio frequencytransmission after conversion to an analog radio frequency signal; and

(d) processing the defined digital codes to provide source-encoding of aset of signals for driving a multiple antenna array, wherein preferablyat least two antennas of the multiple antenna array receive respectivelydifferent signals.

The at least one digital logic unit may perform any two, or all three,functions, in any order, in addition to other functions.

A power amplifier may be provided, adapted to amplify the analog radiofrequency signal to a power of at least 27 dBm, wherein the digitalsignal processor predistorts the digital domain representation to reducean effect of a non-linear distortion of at least the power amplifier.

The digital signal processor may predistort the digital domainrepresentation to increase a spur free dynamic range of the radiofrequency signal by at least 3 dB, and preferably at least 6 dB.

The defined digital codes may be processed to reduce a peak to averagepower ratio by at least 1.0 dB, 1.5, 2.0 dB, 2.5 dB, or 3 dB. At leasttwo sets of defined digital codes may be combined, and the combined setsof digital codes are processed to reduce a peak to average power ratioby at least 1 dB, 1.5, 2.0 dB, 2.5 dB, or 3 dB.

The radio frequency signal may have, for example, a spur-free dynamicrange of at least 40 dB over a bandwidth of at least 19 MHz, andpreferably over 39 MHz or 59 MHz.

The defined digital codes may be processed by the digital logic unit toreduce an amplitude of at least one intermodulation product by at least3 dB, preferably by at least 6 dB, based on a digital model of anon-linear process operating on the analog domain radio frequencysignal.

The digital signal processor may have an input adapted to receive afeedback signal representing a distorted of the output, and wherein thepredistortion is responsive to the feedback signal.

The digital signal processor may comprise an active superconductinglogical device, such as a device implemented using Josephson junctions.

The radio frequency signal may be generated by a power digital to analogconverter, which directly generates an analog radio frequency signalfrom the at least one significant information-bearing component, havingan amplitude of at least 27 dBm, and wherein the power radio frequencysignal has an average power of at least 30 dBm.

The radio frequency signal may have intermodulation products notexpressly defined by the digital signal processor having an amplitude ofat least −40 dB with respect to the radio frequency signal in theabsence of predistortion, and the digital signal processor predistortsthe digital domain representation of the radio frequency signal toreduce an amplitude of at least one intermodulation product by at least3 dB.

The digital signal processor may comprise at least one look-up table,and may provide a plurality of look-up tables operating in tandem. Thedigital signal processor may, for example, comprise a delay elementsignal path, an integrator element signal path, and a differentiatorelement signal path, each signal path having at least one correspondinglook-up table, and a combiner receiving an output of each respectivesignal path, to define a transformation of the digital domainrepresentation of the radio frequency signal.

A radio system is further provided, comprising an optoelectronicreceiver adapted to receive an input from an optic communication mediumcomprising an analog domain representation of a radio frequency signalhaving a frequency of at least 698 MHz; a converter adapted to digitizethe analog domain representation at a sampling rate of at least 1.4gigasamples per second; and a digital signal processor adapted to reducean effective intermodulation distortion inferred to be present in thedigitized analog domain representation based on strong signal componentspresent in the digitized analog domain representation and theirrespective likely intermodulation products. The digital signal processoranalyzes a frequency band greater than about 20 MHz, for example, 40MHz, 60 MHz, 80 MHz, 100 MHz or 120 MHz. The digital signal processorpreferably produces a representation of the analog domain radiofrequency signal having a spur-free dynamic range of at least 40 dB overa bandwidth of at least 19 MHz, or 39 MHz, or 59 MHz. The digital signalprocessor preferably improves the effective spur-free dynamic range ofthe digitized analog domain representation by at least 3 dB, morepreferably by at least 6 dB.

The radio system may further comprise an interface to an antenna adaptedto receive a radio frequency signal of at least 700 MHz; and anelectrooptic transmitter adapted to transmit a signal corresponding tothe received radio frequency signal within the band on the opticcommunication medium.

The digital processor may be adapted to deconvolve a non-lineardistortion of the digitized analog domain representation. Similarly, thedigital processor may be adapted to reverse a non-linear distortion ofthe digitized analog domain representation, said digital signalprocessor comprising at least one look-up table, and for example maycomprise a plurality of look-up tables operating in tandem. The digitalprocessor may further comprise a delay element signal path, anintegrator element signal path, and a differentiator element signalpath, each signal path having at least one corresponding look-up table,and a combiner receiving an output of each respective signal path, todefine a transformation of the digitized analog domain representation ofthe radio frequency signal.

The radio system preferably is implemented using superconducting digitalelectronic logic elements, and for example, the converter and/or digitalsignal processor may comprise at least one active superconductiveelectronic device. The converter may digitize the analog domain radiofrequency signal at a rate of at least 5 gigasamples per second (GSPS),for example 10 GSPS, 20 GSPS or 40 GSPS.

A digital signal processor is provided, comprising an superconductingelectronic circuit receiving as an input a digital signal having atleast two radio frequency components having a center frequency above 698MHz, representing an analog signal having information represented assignal energy within a band, and being subject to non-linear distortion,wherein the input further comprises at least one intermodulation productof the at least two radio frequency components present within the band,said superconducting circuit processing the input to reduce a signalenergy within the band resulting from intermodulation distortion by atleast 3 dB with respect to signal energy corresponding to information,and an output representing at least a portion of the information. Thesuperconducting electronic circuit may comprises at least one look-uptable, and for example may comprise a plurality of lookup tables. Thesuperconducting electronic circuit may further comprise a digitalcorrelator, which may be, for example, a multi-bit autocorrelator. Thedigital signal processor may further comprise a crosspoint switchmatrix, and/or a digital channelizer.

A radio system is provided, comprising a receiver adapted to receive aninput from a communication medium comprising radio frequency signal; aconverter adapted to digitize the signal at a rate of at least 1.4Gigasamples per second to digitally describe a radio frequency waveformhaving frequency components of at least 698 MHz and a bandwidth of atleast 59 MHz; wherein the digitized signal has intermodulation productsrepresented therein having an amplitude of at least −40 dB with respectto the signal; and a digital predistorter adapted to precompensate theinput to reduce an amplitude of at least one intermodulation product byat least 3 dB.

These and other objects will become apparent through review of thedetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary block diagram of a transceiver system of theprior art.

FIG. 2 shows a block diagram of a transceiver system according to thepresent invention.

FIG. 3 shows a block diagram of a preferred embodiment of a transceiversystem according to the present invention, with superconducting digitalelectronics mounted on a cryocooler.

FIG. 4 shows a detailed block diagram of a multichannel transceiveraccording to the present invention, showing multiple signals andmultiple antennas.

FIG. 5 shows a detailed block diagram of a transmitter according to thepresent invention, showing an alternative optical link.

FIG. 6 shows a block diagram of a distorter with proportional, integral,and differential control, that can compensate for nonlinear distortionin amplifiers and optical links, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A block diagram of a modern wireless basestation of the prior art isshown in FIG. 1. This describes a split architecture, which ispartitioned into digital baseband (DBB) processing on the ground (i.e.,within a base station module) and analog RF processing on the tower. Aversion of in-phase and quadrature (IQ) receiver and transmitter areshown here, although other variants known in the art may alternativelybe applied. Thus, it should be understood that FIG. 1 is simplified,and, for example, portions of the electronics may be duplicated.

Consider first the transmitter, where the DBB Synthesizer might, forexample, generate an OFDM (orthogonal frequency domain multiplexed)signal that is up to a few MHz in bandwidth, comprising many narrow bandsignals, properly timed and encoded. This digital signal is then used tomodulate a diode laser or optical interferometer, and the optical signalis coupled into an optical fiber that is sent up the tower. Opticalfibers generally exhibit relatively weak attenuation; the choice offiber would depend on the distance to be covered, which could be quitefar in some cases. The digital optical signal is demodulated at thetower, typically with a photodiode, and the resulting digital electricalsignal recovers the initial DBB signal. This is then sent to a standarddigital-to-analog converter (DAC) to obtain the analog baseband signal,which is then combined with an RF local oscillator in a mixer (with bothI and Q signals in the standard way) to upconvert the signal andgenerate a low-power version of the signal to be transmitted. Thispasses through an appropriate bandpass filter (BPF), is amplified in apower amplifier (PA), and broadcast through an antenna. In typicalwireless systems, the same antenna is used alternately for bothtransmission and reception, with a duplexer to isolate the two systems.

The prior-art receiver system in FIG. 1 goes through the reversetransformations. The very weak received RF signal received through theantenna is first filtered and amplified (using a low-noise amplifier orLNA), and then downconverted, generating I and Q baseband signals. Theseare then digitized at or above the Nyquist rate, and modulate a diodelaser or interferometer. The resulting optical signal is sent off thetower toward the ground station via optical fiber. After regeneration ofthe electrical digital baseband signal, the digital baseband processordecodes and reconstructs the individual sub-channels (channelization)which are then sent along to the user.

The Prior Art IQ Transceiver shown in FIG. 1 includes a Transmitter 24and a Receiver 25. The Transmitter starts with a Base Unit located onthe Ground 22, with Digital Baseband Synthesizer 1, which sends in-phase(I) and quadrature (Q) digital baseband signals to Digital Electro-optic(E/O) Modulators 2. The E/O Modulator transmits I and Q digital opticalsignals at 1-10 MS/s over Optical Fiber 21 to the Tower Unit 23. Thesedigital optical signals are demodulated in Digital Opto-electronicdemodulator 3, which regenerates I and Q digital baseband signals, eachof which is sent to a respective baseband Digital-to-Analog Converter(DAC) 4. The resulting I and Q analog signals are each sent to arespective analog mixer (Upconverter 5), where they are combined withthe output of a Local Oscillator (LO 20), with a 90 degree phase shiftapplied to the input to the mixer for the Q signal. The outputs of theupconverters are then sent to a Bandpass Filter 6, and then combined inan analog combiner (Adder 7) to generate the RF signal to be amplifiedand transmitted. This analog signal is sent to the Power Amplifier (PA8), and the PA output is sent to the Duplexer 10, and then to theAntenna 9 for broadcasting.

FIG. 1 further shows that the Receiver 25 receives a weak RF signal fromthe Antenna 9 on the Tower 23 (when the antenna is not transmitting),and sends it to the Duplexer 10, and then to the Bandpass Filter 11. Thefiltered analog signal is then passed to a Low Noise Amplifier LNA 12,and then split in an analog splitter, with RF signals going to the I andQ receive channels. Each of these is input to a mixer (Downconverter14), where it is combined with the output of a Local Oscillator 19, withthe appropriate 90 degree phase shift for the Q channel. The output ofthe downconverter is digitized by a baseband Analog-to-Digital Converter15 to generate I and Q digital baseband signals, which are thenconverted to digital optical signals in Electro-Optic modulator 16, andsent over Optical Fibers 21 to the Ground Unit 22. These signal arereceived at the ground unit by Digital opto-electronic (O/E) receivers17, and demodulated to obtain the baseband digital signals, which aresent in turn to Digital Baseband Channelizers 18 for recovery of thesignals of interest.

The system of FIG. 1 is well established and reasonably efficient, buthas some serious shortcomings with respect to future evolution ofcellular communications, and communications in general. A key problem isthat the demand for greater bandwidth requires the use of wider channelsand additional bands. That is, the information for a singlecommunication is spread over a broader range of frequencies within achannel, and the various channels are located over a broad range ofspectrum in various bands. Likewise, the same bands may not be availableat all locations for use, but the required economies of scale suggest asingle hardware design for multiple installations, without the need forcustomization. One approach is simply to place more transceiver systemson the towers, but the size, weight, and power consumption of all ofthese is becoming impractical. For example, a key consideration in talltower construction is wind load factors, which is influenced by the sizeand weight of components mounted on the tower. As the size and weightincrease, the tower structure must be correspondingly increased, leadingto increased costs, and in some cases, zoning or aesthetic restrictions.Alternatively, one may combine multiple baseband signals into a singlebroader frequency band, thereby reducing the number of components.However, this requires not only faster digital processing, but alsowider-band analog mixers and PAs. The required PA in particular is asignificant impediment, since increasing the bandwidth tends to increasethe nonlinearity. Nonlinearity generates intermodulation products, whichin turn limit the useful dynamic range of the system, creatinginterference and bit errors. There are well known approaches forlinearizing nonlinear systems, both analog and digital, but these, too,become more difficult as the bandwidth increases.

One aspect of the present invention takes a substantially differentapproach from the prior art system in FIG. 1. First, as shown in FIG. 2and FIG. 3, the system partition between the ground (base station) andthe tower is quite different from that of the prior art system inFIG. 1. The present system generally provides fewer elements on thetower; specifically, frequency shifting and conversion between analogand digital occur in the base station (e.g., on the ground) rather thanin the tower electronics module. Second, the signal being transmitted onoptical fiber between the base station and tower electronics module is abroadband analog signal, rather than a narrowband digital signal, withcorresponding broadband analog modulators/demodulators at both ends.Third, digital processing in the base station ground is carried out atultrafast data rates, taking advantage of the extremely high-speeddigital processing (10-100 GHz) recently made possible by RSFQsuperconducting circuits, though the use of other technologies achievingcorresponding performance is possible. Thus, this architecture is notrestricted to RSFQ, although RSFQ is presently capable of achieving therequired speeds functions and alternative technologies may be unable toachieve the required speeds and functions. And fourth, the presentinvention may incorporate advanced digital domain linearizing componentsthat also operate at these ultrafast data rates, and enable much broaderbandwidths than are feasible using the conventional prior-arttransceivers which seek to perform linearization using traditionalsemiconductor digital processing or analog processing.

FIG. 2 describes the block diagram of one embodiment in accordance withthe present invention, of the Digital-RF™ Transceiver, which includesTransmitter 53 and Receiver 52. Transmitter 53 starts on the Ground 55with a Digital Baseband Synthesizer 31 to generate the digital basebandsignal, which is upconverted using Digital Upconverter 32 and DigitalLocal Oscillator 50 to generate the Digital-RF™ signal, which is adigital domain representation of a radio frequency signal, which isoversampled (i.e., above a Nyquist rate for significant componentsthereof) with respect to the radio frequency signal, at a data rate thatmay be 10-100 GS/s. This signal is then subjected to further digitalprocessing in Digital Predistortion Unit 33, before being converted toan RF analog signal in RF DAC 34. This analog RF signal is then inputinto an Analog Electro-optic (E/O) modulator to generate an analogoptical signal which is sent from the Base Unit on the ground to theTower Unit 54. The optical signal is received by Analog Opto-electronic(O/E) receiver 37, which generates an analog RF electrical signal whichis filtered by Bandpass Filter (BPF) 38, then passed to Power Amplifier(PA) 39 to generate the high-power RF signal to be transmitted. Afterpassing through the Duplexer 40, the signal is sent to Antenna 51 fortransmission. The various analog components are subject to various kindsof distortion, including intermodulation distortion, which result inintermodulation spurs or power peaks which are not directly defined bythe information pattern which leads to the Digital-RF™ signal. TheDigital Predistortion Unit 33 precompensates the datastream for theanticipated distortion, and, for example, produces a high-power RFsignal which has reduced spurs, and in particular it is preferable toprocess the information to reduce the largest amplitude spur.

FIG. 2 further describes the components of Receiver 52, which starts onthe Tower 54 with a weak RF signal received by Antenna 51 (when theantenna is not transmitting), and sent to Duplexer 40, and then toBandpass Filter 41, and to Low-Noise Amplifier (LNA) 42. The amplifiedRF signal is input to the Analog Electro-optic (E/O) modulator whichgenerates an analog optical signal on Optical Fiber 44, which is sentfrom the Tower to the Base Unit on the Ground 55. This analog opticalsignal is received by Analog Opto-electronic (O/E) receiver 45, whichgenerates the RF electrical signal which is sent in turn to RFAnalog-to-Digital Converter (ADC) 46. The ADC generates a fastDigital-RF™ signal which is subject to further digital processing inDigital Postdostortion Unit 47. This is then combined with the output ofDigital Local Oscillator 50 in Digital Downconverter (DDC) 48, theoutput of which is a baseband frequency digital signal which is sent toDigital Baseband (DBB) Channelizer 49.

Consider the transmitter in FIG. 2 in greater detail. Unlike in FIG. 1,the narrowband digital baseband (DBB) synthesized signal is notconverted to analog, but rather is digitally upconverted to a digital-RFsignal at a multi-GHz data rate, using a digital LO rather than ananalog oscillator. Proper IQ upconversion as in FIG. 1 is implied butnot explicitly shown. This digital signal is then processed at amulti-GHz data rate in a digital linearization module operating at radiofrequency data rates (described below in FIG. 6) that predistorts fornonlinear distortion in both the optical link and the power amplifier.This linearization module is capable of superior nonlinear compensation(in terms of suppression of intermodulation interference, for example)to that achievable using more conventional digital-basebandpredistortion at lower data rates. The predistorted signal is thenconverted to an analog RF signal using an ultrafast DAC, which may alsobe implemented in whole or in part, using RSFQ technologies. This RFsignal, in the GHz range, then modulates a linear analog opticalmodulator, and the optical signal is then sent via fiber to the towerelectronics module. At the tower, the optical signal is demodulated witha linear analog photodiode, which is then passed through a bandpassfilter and then amplified in the PA. It is important to note thatalthough FIG. 2 shows a single DBB signal, it is compatible withmultiple DBB signals that can be digitally combined (as shown below inFIG. 5) to form a much broader band signal. Furthermore, FIG. 5 alsoshows an alternative transmitter embodiment wherein the signal ismaintained in digital format through the optical link, as describedfurther below.

For the receiver in FIG. 2, the LNA in the tower electronics moduleneeds to amplify the weak incoming RF signal to a sufficiently highlevel (which may be up to 10 V in amplitude) to drive the EO Modulatorin a linear regime, and the resulting optical signal is sent down to thebase station, where it is demodulated. The resulting RF electricalsignal is then digitized in an ultrafast digitizer, for example usingRSFQ technologies, without prior analog downcoversion. This high-ratedigital signal is then sent to another linearization module operating aradio frequency data rates, for post-distortion, to correct fornonlinear distortion in both the analog amplifier and analog opticallink in the receive chain. Here again, superior linearization can beobtained beyond that achievable using baseband digital processing. Thelinearization modules for transmit and receive are formally similar, butare separately programmable with specific parameters for the relevantdevices in each channel. After linearization, the signal isdownconverted using a digital LO (where again IQ processing is implied),to generate a standard baseband signal that can then be furtherdigitally channelized into multiple narrowband digital signals. Thisimplementation is readily compatible with a much broader signal band, sothat multiple conventional DBB signals can be recovered via the samebroadband receive channel.

FIG. 3 describes a preferred embodiment providing a superconductingactive logic (RSFQ) implementation of the Digital-RF™ Transceiver of,focusing on details of the Ground Unit 55 from FIG. 2. The Transmitter60 starts with Digital Baseband (DBB) Synthesizer 61, which is sent tothe RSFQ unit mounted in a Cryocooler 78. The digital baseband signal isthen up-sampled in a Digital Interpolation Filter 62, and then combinedwith fast Digital Local Oscillator (LO) 64 in Digital Mixer 63,functioning as an upconverter. The resulting Digital-RF signal is thensubject to further digital processing at radio frequencies in theDigital Predistorter 65, and then converted to the analog domain in RFDigital-to-Analog Converter (DAC) 66. This RSFQ Digital-RF™ signal isthen amplified in Pre-Amplifier 67 and sent out of Cryocooler 78 toAnalog Electro-optic (E/O) modulator 68, which sends the analog RFsignal over optical fiber 69 to Tower Unit 54 in FIG. 2.

FIG. 3 further describes the RSFQ implementation of the Receiver unit70, focusing on details of the Ground Unit 55 from FIG. 2. This startswith an analog optical signal coming from Tower Unit 54 in FIG. 2 overoptical fiber 79, and is received by Analog Opto-electronic (O/E)demodulator 71, which generates an analog RF signal that is sent to theRSFQ circuits in Cryocooler 78. This analog RF signal is first digitizedin RF Analog-to-Digital Converter (ADC) 72, and then subjected tofurther digital processing in Digital Post-distorter 73, the output ofwhich is then sent to Digital Mixer 74, functioning as a digitaldownconverter, and combined with the output of Digital Local Oscillator64. The downconverter digital output is sent to Digital DecimationFilter 75, reducing the data rate to the baseband sampling rate. Thisdigital signal is amplified to standard digital levels in Pre-Amplifier76 and sent out of Cryocooler 78, to Digital Baseband Channelizer 77 forfurther baseband processing.

This technology employs integrated circuits comprised of manysuperconducting Josephson junctions, and at present is implementedpreferably using niobium (Nb) as the superconductor, operatingpreferably at temperatures below about 5 K (−268 C). These circuits canbe tested in the laboratory in a Dewar filled with liquid helium at atemperature of 4.2 K, but in the field would be mounted on a cryocoolerinside an evacuated chamber. This cryocooler is a closed-cyclerefrigerator, typically with two or more stages, which may be based on apulse-tube, Gifford-McMahon, or Stirling cycle, and several models aresold commercially that can operate reliably for extended periods as longas electric power remains available. However, the power and weightbudgets for present-day 4K cryocoolers may be too large for placement ona tower, hence a ground-based location in accordance with the presentinvention is preferable.

It is important to point out that RSFQ data converters (ADCs and DACs)are extremely linear, since they are based on a fundamental physicalconstant, the single flux quantum Φ₀=h/2e=2.07 mV-ps, where h isPlanck's constant and e is the charge on the electron. In fact, theinternational defining standard for the volt is now based on arrays ofJosephson junctions, such that a data rate of 100 GHz corresponds to avoltage of 0.207 mV. This high linearity of data conversion is quiteattractive for maintaining linearity in a digital receiver ortransmitter. However, this linearity will be degraded by nonlineardistortion in the amplifiers and optical converters. Fortunately, thelinearity can be restored through the use of appropriate “inversedistortions”, also possible using RSFQ circuits.

It should be noted that various technological implementations for thevarious components are possible, however, if a cryocooler is requiredfor any RSFQ component, the use of additional RSFQ components isgenerally efficient.

Also shown in FIG. 3 are digital circuits required for digital rateconversion: a digital interpolation filter for up-sampling and a digitaldecimation filter for down-sampling. These would also be implemented (atleast in part) using ultrafast RSFQ circuits. Furthermore, since RSFQcircuits are a very low-power, low-voltage technology, pre-amplifiersare needed to increase the voltage level from ˜1 mV to ˜1V to interfacewith external conventional electronics. For the transmitter, thispre-amplifier may be integrated with the DAC, as for example in thepublished patent application entitled “Multibit Digital Amplifier” (U.S.patent application Ser. No. 12/002,592, expressly incorporated herein byreference). For the receiver, where the digital pre-amplifier interfaceswith the DBB channelizer module, efficient, high-gain, nonlinearswitching amplifiers could be used, since analog-grade linearity is notrequired.

The pre-amplifiers and optical converters can operate at standardtemperatures, but in some cases the device noise may be reduced bycooling, typically to temperatures above the 4K operating temperature ofthe RSFQ circuits. Since most 4K cryocoolers are designed with thermalstages at an intermediate temperature of, say, 70 K, integration ofthese devices with the RSFQ circuits may be directly obtained within asingle system having multiple temperature modules. A further advantagemay be that optical fibers may be provided that carry less heat into thecryogenic environment than a metallic coaxial line with a similarbandwidth, and thus reduce the heat load on the low temperature portionscryocooler.

Similarly, although the low noise amplifier (LNA) and band pass filter(BPF) of the receiver in FIG. 2, mounted in the tower electronicsmodule, can operate quite well at ambient temperatures, the noise couldbe further reduced by cooling to temperatures of order 70 K. 70Ksingle-stage cryocoolers are available which are compact, efficient, andvery reliable, and may thus be employed. Further, high-performance BPFbased on “high-temperature superconductors” (such as YBa₂Cu₃O₇, whichsuperconducts below 90 K) are commercially available and will alsooperate well at 70K. Tower-mounted analog receiver assemblies comprisinga 70K cryocooler with a superconductor BPF and a semiconductor LNA arealready commercially available, and thus may be employed.

FIG. 4 shows the flexibility of a transceiver in accordance with anembodiment of the present invention which may be applied to amulti-channel, multi-band system, through the use of an RSFQ switchmatrix (See, U.S. Pat. No. 7,362,125 and published application Ser. Nos.11/966,889, 11/966,897, 11/966,906, and 11/966,918, each of which isexpressly incorporated herein by reference). For clarity, this switchmatrix is not shown in the optical links of the transceiver.

FIG. 4 describes the block diagram of Digital-RF™ multi-channel,multi-band transceiver, comprised of Receiver 80 and Transmitter 90.FIG. 4 focuses on Ground Unit 55 of FIG. 2, and does not show thedetails of the Tower Unit 54 or the optical link (35, 36, 37, 43, 44,45). It is further indicated in FIG. 4 that in one implementation of theMultichannel Transceiver, the Digital-RF™ processing would be carriedout in Superconducting Electronics (SCE), and the digital basebandprocessing would be carried out using more conventional Room-TemperatureElectronics (RTE).

The Receiver 80 in FIG. 4 starts with a plurality of m antennas 81 whichmay receive m RF signals from m bands, each signal being sent to arespective Analog-to-Digital Converter 82. The Digital-RF™ output ofeach ADC is then sent to an m×n Digital Switch Matrix 83, to generate ndigital-RF outputs being sent to n respective Digital Channelizer Units84. It is noted that m and n need not be equal, and a single antennasignal may be presented to any number of processing devices. Likewise,some antennas may be dormant for more certain periods, and not connectedfor processing. Typically, the Digital Switch Matrix 83 is notconfigured to receive multiple inputs from various antennas; however, aprocessing device, such as a MIMO processor, may have multiple ports onthe matrix which allow it to process multiple antenna signals. EachDigital Channelizer Unit comprises a Digital I&Q (In-phase & Quadrature)Downconverter 85 and a Digital I&Q Decimation Filter 86. The DigitalLocal Oscillator (LO 64 in FIG. 3) is implied but not shown. TheDecimation Filter reduces the digital data rate to the baseband samplingrate, and the digital baseband signal is passed to the Baseband DigitalSignal Receive Processor 89 for further channelization, demodulation,decoding, despreading, etc. When MIMO technologies are employed, thesignals from a plurality of antennas are not treated orthogonally, andmust be processed together.

FIG. 4 further describes the Transmitter 90. The Baseband Digital SignalTransmit Processor 99 generates n baseband digital signals, which arepassed to n Digital Transmitter Units (DTU) 95. Each DTU is comprised ofa Digital I&Q Interpolation Filter 97 (to increase the digital samplingrate) and Digital I&Q Upconverter (with the Digital LO implied but notshown). The resulting n Digital-RF signals are sent to an n×m DigitalSwitch Matrix 94, and m digital-RF signals are sent to m respectivetransmit chains. Each transmit chain includes a DigitalPredistorter/Digital-to-Analog Converter Unit 93, Power Amplifier 92,and Antenna 91. In the case of MIMO, the Digital-RF™ signals for aplurality of antennas must be coordinated, for example at the level ofthe Digital Transmitter Units 95.

The antennas for the various bands might be located on the same tower oron different towers. This may also include systems with multi-input,multi-output signals in the same frequency range, known as MIMO. Withinthe spirit of “software-defined radio” or “cognitive radio”, the switchmatrix may be dynamically reprogrammed to redistribute signals accordingto the current traffic and availability of the various bands.

Likewise, more sophisticated antenna systems may be provided, such aselectronically steerable antenna arrays, conformal antenna arrays,synthetic aperture antennas, and the like. In particular, the digitaland analog processing is not limited to traditional cellular radioimplementations.

MIMO systems exploit the spatial separation of antennas operating in thesame band and having overlapping native coverage areas to extract orconvey information, which can lead to a reduction in interference,increased channel information carrying capacity, or other advantages.Spatial multiplexing is a transmission technique in MIMO wirelesscommunication to transmit independent and separately encoded datasignals, called streams, from each of the multiple transmit antennas.Therefore, the space dimension is reused, or multiplexed, more than onetime. If the transmitter is equipped with N_(t) antennas and thereceiver has N_(r) antennas, the maximum spatial multiplexing order (thenumber of streams) is N_(s)=min(N_(t), N_(r)), if a linear receiver isused. This means that N_(s) streams can be transmitted in parallel,leading to an N_(s) increase of the spectral efficiency (the number ofbits per second and per Hz that can be transmitted over the wirelesschannel). Therefore, in accordance with the present technology, multipleantennas and mast-mounted electronics modules, with spatial separationand overlapping beam patterns and frequency bands of operation, which inaccordance with aspect of the invention may be simplified with respectto generally used electronics modules, are employed. This may beparticular advantageous with WiMax (IEEE 802.16), and 3G or latercellular technologies. Typically, the multiple radio frequency signalsor their digital representations, are communicated as wavelengthdivision multiplexed (WDM) signals on a single optic fiber, thoughmultiple optic fibers may also be employed. Preferably, the signals arenot frequency translated in an analog domain for modulation in a singleoptic channel, since this frequency translation generally degrades theSFDR and may produce distortion and bandwidth limitations on the signal.

In an open-loop MIMO system with N_(t) transmitter antennas and N_(r)receiver antennas, the input-output relationship can be described asy=Hx+n, where x=[x₁, x₂, . . . , x_(Nt)]^(T) is the N_(t)×1 vector oftransmitted symbols, y, n are the N_(r)×1 vectors of received symbolsand noise respectively and H is the N_(r)×N_(t) matrix of channelcoefficients. In a closed-loop MIMO system the input-output relationshipwith a closed-loop approach can be described as y=HWs+n, where s=[s₁,s₂, . . . , s_(Nn)]^(T) is the N_(s)×1 vector of transmitted symbols, y,n are the N_(r)×1 vectors of received symbols and noise respectively, His the N_(r)×N_(t) matrix of channel coefficients and W is theN_(t)×N_(s) linear precoding matrix.

A precoding matrix W is used to precode the symbols in the vector toenhance the performance. The column dimension N_(s) of W can be selectedsmaller than N_(t) which is useful if the system requires N_(s)(≠N_(t))streams because of several reasons, for example, if either the rank ofthe MIMO channel or the number of receiver antennas is smaller than thenumber of transmit antennas.

Preferably, these transformations may be performed by the digital signalprocessor processing the information at radio frequency sample rates,thus avoiding introduction of distortion due to analog downconversion orthe like. In the transmit signal processing, the digital signalprocessor may perform a beamforming operation to control a radiationpattern of a set of common information transmitted by a plurality ofantennas, this increasing an effective signal power in spatial regionsof interest, while reducing an effective signal power where it is notneeded. Likewise, the transmitted signals may be precoded with differentinformation patterns on different antennas, especially in a multi-userenvironment (e.g., multiple remote or mobile receivers receivinginformation from a central antenna array). The receiver's task typicallyinvolves, in part, a beamforming task, as well as compensation formultipath effects and possible other distortions, through a plurality ofantennas having overlapping native beam patterns and common operatingfrequency band. The MIMO processing therefore involves compensation fordelay, matrix transformation, signal correlation and autocorrelation(e.g., for multipath echo detection and processing) and the like. Thesefunctions are each available in high speed digital processors, such asRSFQ circuits. See, e.g., U.S. Pat. No. 7,440,490, expresslyincorporated herein by reference. See also, Wikipedia.

To achieve MIMO from a conventional system, several technologies areavailable:

-   -   Beamforming is known as antenna array signal processing, where        every antenna elements are separated from its nearest element by        half of the transmit signal wavelength.    -   Space-Time Coding/Space-Time Processing performs antenna        diversity with multiple antennas at either transmitter or        receiver side or both sides, where every antenna elements are        separated from its nearest element by around 4 to 10 times the        wavelength to keep the signal through each multi-path        independent. The distance between two adjacent antenna elements        is relying on the angular spread of the beam signal.    -   SDMA is a common and typical multiple input multiple output        scheme in cellular wireless systems. SDMA is often referred to        as simply a MIMO system since the half port of a SDMA system        also consists of multiple users.    -   Spatial multiplexing is performed by multiple antennas equipped        at both a transmitter and a receiver front end.    -   Cooperation are known as network MIMO systems, distributed MIMO        systems or virtual antenna array systems. Mobile devices use the        partnered mobile devices' antennas, antenna arrays, or antenna        elements as virtual antennas.    -   Combinations of above techniques, etc.    -   use an existing techniques with enhanced PHY capabilities, e.g.,        a 16×16 array configuration.    -   use special MIMO algorithms such as precoding or multi-user        scheduling at the transmitter.    -   cooperative antenna MIMO.    -   virtual antenna MIMO.    -   Intelligent spatial processing, e.g., RADAR beamforming.

Multi-user MIMO can leverage multiple users as spatially distributedtransmission resources, requiring significant signal processing.Multi-user MIMO can generalized into two categories: MIMO broadcastchannels (MIMO BC) and MIMO multiple access channels (MIMO MAC) fordownlink and uplink situations, respectively. Single-user MIMO can berepresented as point-to-point, pairwise MIMO.

Many Antennas is a smart antenna technique, which overcomes theperformance limitation of single user MIMO techniques. In cellularcommunications, the number of the maximum considered antennas fordownlink is 2 and 4 to support LTE and IMT-A requirements, respectively,though arrays of 8-64 or more antennas have been proposed. With largeantenna arrays, such techniques as New SDMA: MU-MIMO, Network MIMO(COMP), Remote radio equipments; New beamforming: linear beamformingsuch as MF, ZF and MMSE and nonlinear beamforming such as THP, VP, andDPC; New antenna array: direct, remote and wireless antenna array;Direct antenna array: linear and 3D phased array, new structure array,and dynamic antenna array; and Remote and wireless antenna array:distributed antenna array and cooperative beamforming may be employed,alone or in combination or subcombination.

FIG. 5 shows several DBB signals separately upconverted (with differentLOs), which are then combined digitally to generate the signal to betransmitted. Three such DBB signals are shown for simplicity, but manymore could be included, since the present approach is compatible withextremely broadband signals, in principle all the way to the carrierfrequency. In order to combine a plurality of communication streams at aradio frequency data rate, a digital combiner may be employed. See, U.S.2008/0101501, expressly incorporated herein by reference.

FIG. 5 describes an alternative Digital-RF™ Optical TransmitArchitecture. Several Digital Baseband Synthesizers (101, 102, 103) areupconverted in respective Digital Upconverters (104, 105, 106), whereDigital Local Oscillators are implied but not shown. The resultingDigital-RF™ signals are combined in a Digital Combiner Unit 107,essentially a fast digital adder. The combined, broadband Digital-RF™signal is then subjected to further digital processing at RF in DigitalCrest Factor Reduction Unit 108, Digital Predistortion Unit 109, andDigital Encoder Unit 110 (which may convert from an N-bit signal to anoversampled 1-bit signal). In a preferred embodiment, Units 104-110would be expected to be carried out using RSFQ superconductingcomponents mounted on Cryocooler 111. The resulting Digital-RF™ signalis then amplified in Digital Amplifier 112 and maintained in pulseformat. This Digital-RF™ signal is then modulated onto Digital OpticalLink 114 using Digital Electro-Optical modulator 113. The optical signalis received on the Tower Unit by Digital Opto-Electronic Demodulator115, and the resulting digital electrical signal is filtered in BandpassFilter (BPS) 116, thus converting the oversampled pulse train to ananalog RF signal, which is passed to Power Amplifier 117 and transmittedby Antenna 118.

FIG. 5 thus displays several blocks of digital signal processing atradio frequencies, following the digital-RF™ combiner. These include ablock for Digital Crest Factor Reduction (DCFR), Digital Predistortion,and Digital Encoder. These blocks indicate sequential processing of theDigital-RF™ signal, although in practice they may be appropriatelyintegrated in a single digital-RF processing unit. The need for DCFRreflects the observation that combination of multiple signals atdifferent frequencies will inevitably increase the peak-to-average powerratio (PAPR), also known as the Crest Factor (CF). This increase in PAPRwill tend to reduce the power efficiency of the power amplifier (PA) onthe tower unit, which is undesirable. Several techniques are known inthe prior art for reducing the PAPR without excessive distortion of thesignal, although these prior art techniques are applied to the basebandsignal, or equivalently to the envelope of the RF signal. In the diagramin FIG. 5, digital processing is applied directly to the full digital-RFsignal, which provides additional flexibility for reducing PAPR inbroadband signals. The DCFR is followed by the Digital Predistorter, asdescribed earlier in reference to FIG. 2, and the Digital Encoder, whichis further described below.

FIG. 5 also shows a variant approach to the optical link for thetransmitter. Recall that FIG. 2 discloses the use of broadband analogelectro-optical converters, in contrast to the baseband digital link ofthe prior art. This also requires a fast radio frequency capable DAC. Inan alternative approach, a multibit digital signal may be converted to asingle-bit oversampled signal using a digital delta-sigma modulator oranother digital encoder such as that disclosed in U.S. Pat. No.6,781,435, expressly incorporated herein by reference. Such a single-bitsignal, at a very high rate (possibly approaching 100 GHz) might be usedto drive a digital E/O modulator, which would then regenerate anoversampled digital domain radio frequency rate signal on the tower.This could be readily converted to an analog signal simply by filteringout the high-frequency noise, as is customary for delta-sigma DACs. Theadvantage of such a digital link is that is not subject to nonlineardistortion. A possible disadvantage is that it requires an enormousbandwidth on the E/O and O/E converters. In another variant (not shownin FIG. 5), each bit of the multibit digital domain radio frequency ratesignal could be separately transmitted to the tower, and combined in amulti-bit digital amplifier such as that disclosed in U.S. patentapplication Ser. No. 12/002,592, expressly incorporated herein byreference. This requires, for example, adding multiple digital domainradio frequency rate amplifiers and a precision analog RF combiner onthe tower, but the digital sample rate would be much reduced, relaxingthe requirements on the digital E/O and O/E converters.

Note that the technology of digital communication on optical fiberslends itself to sending multiple independent digital signals on the samefiber between the base unit and the tower unit. This is accomplished bymultiplexing two or more signals at slightly different wavelengths, andis known as wavelength division multiplexing, or WDM. These digitalsignals at different wavelengths could represent multiple bits of thesame digital signal, or alternatively a deserialized bit stream, or evencompletely independent signals. The multiple signals would then bedemultiplexed at the receiving end of the fiber, without significantcrosstalk or interference among the signals. The decision on how topartition multiple digital signals among optical fibers may bedetermined in specific cases by considerations of performance and cost.

FIG. 6 provides a block diagram of a digital domain Linearizer,employing a plurality of Lookup Tables, which operates at RF onDigital-RF™ Input signal 120 to generate a pre-distorted (orpost-distorted) Digital-RF™ output signal 130. The linearizer comprisesthree parallel channels, reflecting a control system with proportional,integral, and differential control. Each channel has its own LookupTable Memory (123, 125, 127). The differential channel is generated byDigital Differentiator 126, and the integral channel by DigitalIntegrator 122. All three channels are combined using Digital Combiner128, with Digital Delay 124 included to maintain proper pipelining andsynchronization among parallel channels.

This system operates not on the amplitude and phase of the signal, as inconventional linearizers, but on the sampled RF signal itself. For thisreason, it is not limited to narrowband signals, but rather accounts forharmonics of the RF signal. Since strong nonlinearities generateharmonics, this digital domain radio frequency rate approach is usefulin broadband systems where the intermodulation products extend much morewidely than the signal itself. FIG. 6 shows that each linearizer (foreither predistortion or postdistortion) could include up to threedistinct lookup tables (LUT), corresponding in principle to theProportional, Integral, and Differential components of the PIDcontroller. The proportional LUT would generate a distorted outputreflecting the instantaneous value of the digital domain radio frequencyrate waveform, while the differential LUT would reflect slew-rate (orfrequency-dependent) effects, and the integral LUT would reflect issuesof average applied power. These LUT are based on detailed model(s) ofthe relevant amplifier or optical links, but the values of the LUTscould be periodically refreshed, possibly by a self-adaptive mechanism,in a dynamic control system. Because the outputs of the PID LUTs aredigitally combined (added) at the output, all parallel circuits arepreferably properly synchronized by proper pipelining, with delayelements added as needed.

One consideration in addressing linearity issues is the tradeoff betweenlinearity on the one hand and efficiency (as well as cost, weight, andsystem complexity) on the other hand. It is well known that one mayimprove linearity in PAs by selecting a PA with excess capacity, andoperating well below saturation capacity in the linear regime, aprocedure known as “backoff”. A similar constraint may be present inhigh-linearity optical converters. The advantage of using digital domainradio frequency rate linearizers is that a system designer has theflexibility to select devices and operate them in a regime that may berelatively nonlinear, while correcting these nonlinearities with digitalprocessing. Indeed, a component with well characterized non-linearitiesmay be used in preference to one which is more linear, but for which theresidual non-linearities are more complex, or less characterized, orless predictable. This will permit an increase in system efficiency withdecreased cost and weight of hardware on the tower.

A related issue is associated with combined RF signals having a largepeak-to-average-power ratio (PAPR), which is inevitable in widebandsignals based on orthogonal frequency-domain multiplexing (OFDM) andsimilar approaches. A large PAPR is undesirable in that it may lead touse of PAs (and optical converters) with excess dynamic range andcapacity, which can be inefficient and expensive. The prior art hasidentified algorithms, operating on baseband signals, that can decreasePAPR somewhat, typically in exchange for some nonlinear spectralbroadening. However, in accordance with an embodiment of the presentinvention, the digital domain radio frequency rate linearizers may beselectively programmed to reduce the PAPR by operating directly on thedigital signal. This may permit an improved optimization and tradeofffor very broadband RF signals.

The prior art has also identified several multi-amplifier approaches forincreasing the efficiency of PAs. These include Doherty amplifiers,polar modulation (envelope elimination and tracking), and outphasingamplifiers (linear amplification with nonlinear components, or LINC).While the examples provided herein demonstrate single-channel PAs, thisin no way excludes the use of one or more of these more sophisticatedamplifier techniques, which would provide correspondingly improvedperformance. Similar digital domain radio frequency rate techniquesincluding digital domain radio frequency rate linearization andbroadband optical links may be applied by one skilled in the art tosystems comprising these alternative amplifier designs.

What is claimed is:
 1. A communication system, comprising: anoptoelectronic receiver configured to receive an optic signal conveyedfrom an electrooptic transmitter to an optic communication medium,comprising an analog domain representation of a radio frequency signalhaving frequencies above 698 MHz modulating the optic signal, and toproduce an electrical analog domain signal corresponding to radiofrequency signal frequencies above 698 MHz, including at least oneintermodulation distortion product of the radio frequency signal; adigitizer configured to quantize the electrical analog domain signal toa digital stream having a sampling rate of at least 1.4 gigasamples persecond; and a digital signal processor configured receive the digitalstream and digitally reduce a representation of an amplitude of at leastone intermodulation distortion product, dependent on a model of theintermodulation distortion properties of the at least one of theoptoelectronic receiver and the electrooptic transmitter, and to producea processed signal output.
 2. The communication system according toclaim 1, wherein the digital signal processor is configured to reduce arepresentation of an amplitude of at least one intermodulationdistortion product over a range of a frequency band greater than about20 MHz.
 3. The communication system according to claim 2, wherein thedigital signal processor is configured to reduce a representation of anamplitude of at least one intermodulation distortion product over arange of a frequency band greater than about 60 MHz.
 4. Thecommunication system according to claim 1, wherein the digital signalprocessor is configured to improve a effective spur-free dynamic rangeof the digitized analog domain representation by at least 3 dB.
 5. Thecommunication system according to claim 4, wherein the digital signalprocessor is configured to improve a effective spur-free dynamic rangeof the digitized analog domain representation by at least 6 dB.
 6. Thecommunication system according to claim 1, further comprising: aninterface configured to receive a radio frequency signal having afrequency above than 698 MHz; and the electrooptic transmitterconfigured to produce the optic signal conveyed in the opticcommunication medium, comprising an analog domain representation of theradio frequency signal from the interface having frequencies above 698MHz modulating the optic signal.
 7. The communication system accordingto claim 1, wherein the digital processor comprises a plurality oflookup tables, and is configured to deconvolve a non-linear distortion.8. The communication system according to claim 1, wherein the digitizercomprises a Josephson junction configured to digitize the analog domainsignal at a rate of at least 40 GHz.
 9. The communication systemaccording to claim 1, further comprising a digital channelizerconfigured to segregate a broadband signal represented in the processedsignal output into a plurality of different independent communicationchannels.
 10. A communication method, comprising: receiving an opticsignal conveyed from an electrooptic transmitter to an opticcommunication medium to an optoelectronic receiver, comprising an analogdomain representation of a radio frequency signal having frequenciesabove 698 MHz modulating the optic signal; producing an electricalanalog domain signal corresponding to radio frequency signal frequenciesabove 698 MHz, including at least one intermodulation distortion productof the radio frequency signal; quantizing the electrical analog domainsignal to a digital stream having a sampling rate of at least 1.4gigasamples per second; and digitally reducing a representation of anamplitude of at least one intermodulation distortion product, dependenton a model of the intermodulation distortion properties of the at leastone of the optoelectronic receiver and the electrooptic transmitter. 11.The method according to claim 10, wherein a representation of anamplitude of at least one intermodulation distortion product is reducedover a range of a frequency band greater than about 20 MHz.
 12. Themethod according to claim 10, wherein a effective spur-free dynamicrange of the digitized analog domain representation is improved by atleast 3 dB.
 13. The method according to claim 10, further comprising:receiving a radio frequency signal having a frequency above than 698MHz; and producing the optic signal conveyed in the optic communicationmedium, comprising an analog domain representation of the received radiofrequency signal having frequencies above 698 MHz modulating the opticsignal.
 14. The method according to claim 10, wherein said model isrepresented by data stored in a plurality of lookup tables, which isused to deconvolve a non-linear distortion.
 15. The method according toclaim 10, further comprising segregating a broadband signal representedin the processed signal output into a plurality of different independentcommunication channels with a digital channelizer.
 16. A method ofreducing a crest factor in a digital representation of an analog radiofrequency signal which is adapted for amplification through a poweramplifier and transmission as a radio frequency signal through anantenna, comprising: receiving a plurality of oversampled digitalrepresentations of radio frequency signals having different frequencies;determining that a combination of the plurality of oversampled digitalrepresentations of radio frequency signals having different frequencieswill produce a first composite signal having an excessivepeak-to-average power ratio and thereby reduce an efficiency of thepower amplifier; digitally modifying the combination of the plurality ofoversampled digital representations of radio frequency signals havingdifferent frequencies to produce a second composite signal having areduced peak-to-average power ratio with respect to the peak-to-averagepower ratio of the first composite signal while meeting at least onepredetermine signal distortion criterion; and outputting the secondcomposite signal.
 17. The method according to claim 16, wherein theplurality of oversampled digital representations of radio frequencysignals having different frequencies are representative of digitalsignals having a bit rate above 1.4 GHz.
 18. The method according toclaim 16, further comprising digitally predistorting the secondcomposite signal to reduce an intermodulation distortion present in anoutput of the power amplifier.
 19. The method according to claim 18,further comprising converting a multibit parallel representation of thesecond composite signal into a single bit serial representation digitalstream.
 20. The method according to claim 19, wherein the single bitserial representation digital stream has a bit rate greater than 1.4GHz, and a power insufficient to directly drive an output device,further comprising digitally amplifying a power of the single bit serialrepresentation digital stream to a power level sufficient for directlydriving the output device.